1. Field of the Invention
This invention relates to a method and an apparatus useful in the chemical etching of silicon.
2. Description of the Prior Art
Methods for physical abrasion and chemical etching of silicon are well-known in the prior art. During the manufacture of silicon semiconductor devices, silicon wafers are put through a variety of chemical processes. These processes form the desired electronic circuits on the front side 1 of the silicon wafer 2 of FIG. 1. At the same time, on the backside of the wafer an undesired semiconductor diode junction 5 is formed, as well as an undesired non-uniform oxide layer 3. Oxide layer 3 is then ground off in a back-lap process to expose bare silicon on the back surface 4 of wafer 2. By exposing bare silicon on the back surface 4, improved electrical contact can be made to back surface 4. In addition, an improved surface is made available for attaching individual dice contained on wafer 2 to a package or substrate. Back-lap is also performed to control the thickness of finished wafer 2.
During the back-lap process, a large amount of silicon debris is formed on back surface 4 of wafer 2. This debris must be removed in order to realize the benefits for which the back-lap operation was performed. Several different methods of removing this debris are used. Various washes, including ultrasonic baths, have been used in the prior art. Acid etches have also been used in the prior art to remove this debris, as well as for other purposes, including the preparation of the back side of the wafer for deposition of metal.
Back-lapping also creates a stress layer on the back side of the wafer due to the abrasion. This stress layer, containing many stress multipliers, promotes the destruction of semiconductor dice contained on the wafer due to cracking. This problem is aggravated by the high temperatures used to attach the die to a package or a substrate, and the stress generated due to different coefficients of expansion of the die and the package or substrate during temperature changes. Chemical etching can also be used to remove this stress layer from the back side of the wafer.
Perhaps the most generally preferred method of etching silicon is the use of an etchant solution containing hydrofluoric, acetic, and nitric acids. Such a method is described in "Chemical Etching of Silicon", by Schwartz & Robbins, which appeared in the Journal of Electrochemical Society: Solid State Science & Technology, December 1976, page 1903. This three-acid system provides a two-step etch. The nitric acid oxidizes silicon to silicon dioxide, and the hydrofluoric acid dissolves the silicon dioxide from the surface of the wafer. The rate of reaction (etch rate) increases with increased temperature and increased agitation. The etch rate is also dependent on the precise composition of the etchant solution, as has been widely reported in the literature, and the amount of dissolved silicon in the solution.
During the back-lap process, means of protecting the electronic circuitry formed on the front side of the wafer is required. One prior art method is shown in FIG. 2. Wafers 71 are cemented to backing plate 70 utilizing a special wax. Backing plate 70, together with the wax, protects circuits formed on the front side of the wafer, while allowing the back surface of wafer 71 to remain exposed during the back-lap and chemical etching operations. A second prior art technique of protecting circuits formed on the front side of a semiconductor wafer is through the use of a plastic tape, such as "Etching Discs" manufactured by the Cellotape Company of Sunnyvale, California. This tape contains an adhesive, which allows the tape to be firmly affixed to the front side of a wafer to be protected. This plastic tape is impervious to the acids used to etch silicon; therefor, it is also used to protect circuits formed on the front side of the wafer during acid etches of the back side of the wafer.
A major disadvantage of using a backing plate to protect the front side of the wafer during back-lap and etching is that its use is rather time consuming. Furthermore, due to the size of the backing plate, relatively few wafers may be etched in an acid bath of any given size. A disadvantage with the tape method of protecting circuitry contained on the front side of a wafer during etching is that the adhesive holding the tape to the front surface of the wafer is functional in acid baths only at rather low temperatures. For best results, the acid bath should be maintained below 20.degree. C. in order to ensure that the tape will function. Both of these prior art methods severely limit the throughput of wafers at the etch operation. Typically, only about 7 wafers are etched at once in a prior art system containing approximately 15 gallons of etchant.
One problem with the prior art etching process is that it generates heat thereby raising the temperature of the etching solution. When this temperature rises above 20.degree. C., the tape used to protect the previously formed circuitry on the front of the wafer becomes less effective thereby reducing the number of good dice obtained from the wafer.
An experimental value was calculated for the heat of solution of silicon in an etching solution containing acetic, nitric and hydrofluoric acids. This heat of solution was found to be approximately 21 kilocalories per gram of silicon dissolved in the etchant. This very high heat of solution is indicative of the fact that the reaction between the acid etch and silicon is highly exothermic, thereby causing a rapid increase in the temperature of the acidic etch solution and thus limiting the etch solution to use with wafers whose front side circuitry has been protected with plastic tape.
The heat removal rate from the etching system can be expressed as ##EQU1## where R=the heat removal rate, M/t=the mass flow of the heat transfer medium per unit time, C.sub.p =the heat capacity of the heat transfer medium, .DELTA.T=the temperature change of the heat transfer medium from the entrance to the exit of the cooling coil or cooling chamber.
Prior art methods of removing this heat generated during the chemical etch of silicon had been primarily through the use of cooling jackets surrounding the etching solution, or cooling tubes immersed within the etching solution. An etchant tank having a cooling jacket surrounding the etchant is shown in FIG. 3. Etchant tank 25 is comprised of inner wall 18 and outer wall 17, thus forming cooling jacket 15. Inlet 20 and outlet 21 are connected to provide means for a coolant to enter and flow through cooling jacket 15, thus cooling the solution contained within region 19. In a similar fashion, a cooling coil may be constructed of material immersed in the etchant. The surface of the immersed cooling coil exposed to the etchant must be impervious to physical and chemical attack by the etchant. Materials such as stainless steel, titanium, or various plastics, such as Kynar, sold by the Penwalt Corporation, or Teflon, sold by Dupont, are suitable for this purpose.
Prior art methods of cooling silicon etching solutions are limited to utilizing water as the cooling medium, or utilizing a refrigerant, together with an external refrigeration apparatus. Such prior art coolants are rather expensive and inefficient in removing large quantities of heat so as to maintain the etching solution at the desired temperature of approximately 10.degree. C., due to the relatively small temperature difference (.DELTA.T) between the chemical etch and the coolant.